Profile and corrugated plastic pipe used in drainage, irrigation, storm sewer and sanitary sewer applications is produced from high density polyethylene (HDPE). A typical pipe composition comprises HDPE resin having a melt-flow index of about 0.15 to 0.4 blended with a small amount of carbon black for ultraviolet light protection. As used herein, the melt flow index is intended as an equivalent expression to the melt flow rate expressed as grams per 10 minutes at 190xc2x0 C.
The Departments of Transportation (DOT) of many states of the United States require plastic pipe used for DOT projects to meet American Association of State Highway Transportation Officials (AASHTO) standards that include American Society of Testing Materials (ASTM) standards. Historically, corrugated pipe manufacturers used monomodal medium molecular weight (MMW)-HDPE to meet the required cell classifications in ASTM D-3350 for melt index, density, flexural modulus, tensile strength, bent strip environmental stress crack resistance (ESCR) and notched izod. The modality of the polymer (e.g., monomodal, bimodal, trimodal, and the like) relates to the number of peaks in a molecular weight distribution curve. In some cases, blends of virgin, recycled, reprocessed and/or scrap polyethylenes were used to achieve the required properties for specific cell classifications. Although these methods were effective for satisfactory short-term physical properties of the resulting pipe, in the longer term in the field the pipe had poor slow crack growth properties.
To address this problem, the AASHTO standards were recently modified to require a minimum stress crack resistance of 24 hours for corrugated and profile pipe as determined by the newly required Notched Constant Tensile Load (NCTL) test, and the previous ESCR test standard was eliminated. The replacement of the ESCR requirement by the NCTL requirement was done to ensure improved pipe performance related to long term stress cracking when the pipe is buried. However, it was found that, for the most part, the previous HDPE compositions did not meet the NCTL standard.
In response to this problem, specialty grade HDPE resins were developed by resin manufacturers. These resins have a narrow molecular weight distribution (MWD) and are produced by multistage polymerization resulting in a bimodal or multimodal HDPE that satisfies the NCTL standard and other AASHTO requirements for corrugated and profile pipe. However, these specialty resins are significantly more expensive than the standard grades typically used in the pipe manufacturing industry.
In an attempt to lower costs and still meet the required standards for HDPE pipe compositions, melt blends of lower priced commodity grades of HDPE were prepared in two- and three-material blend combinations and then tested for the cell classification properties. For example, monomodal high molecular weight (HMW)-HDPE copolymers, such as those used for blow-molded 55 gallon drums, were melt blended with monomodal MMW-HDPE copolymers, such as those used for blow-molded detergent bottles. However, the resulting HDPE blend had NCTL values that were less than 24 hours, although the melt index, density, notched izod, flexural modulus and tensile strength were within the AASHTO specifications. Therefore, this attempt was not successful in providing a viable substitute for the high cost specialty resins for profile and corrugated pipe applications.
In another attempt, three-material melt blends employing commodity linear low density polyethylene (LLDPE) resins, such as those used for drycleaning bags, in combination with commodity HMW-HDPE resins and/or homopolymer high density polyethylene (H-HDPE) resins, as described above, were prepared. The resulting composition met all of the AASHTO standards, including the NCTL standard. These melt blended compositions are the subject of co-owned, co-pending U.S. patent application Ser. No. 10/022,706.
In order to reduce costs and to accommodate manufacturing systems that are limited to producing two-material blends, there is an ongoing need to provide commodity resin combinations that result in melt blends that meet the AASHTO standards for profile and corrugated pipe.
The present invention provides HDPE two-material melt blend compositions for use in the manufacture of profile and corrugated pipe that meet all of the AASHTO standards, including the NCTL properties, and reduce or eliminate the necessity of using expensive specialty-grade HDPE resins.
An important component of the blends is a commodity film-grade bimodal HMW-HDPE resin that is commercially available for use in the manufacture of T-shirt bags, trash can liners and other heavy duty bags. It was unexpectedly discovered by independent testing of these resins by the inventor, that some of these bimodal HMW-HDPE resins inherently have a NCTL property of about 200 hours to as high as 1500 hours or greater. It was further unexpectedly discovered that combining a high NCTL film grade bimodal HMW-HDPE resin with either a commodity homopolymer MMW-HDPE resin, such as those used for milk, water or fruit juice bottles, or a commodity copolymer MMW-HDPE resin, such as those used for bleach or detergent bottles, produced a melt blended two-material HDPE composition that meets or exceeds all the AASHTO requirements for profile and corrugated pipe and pipe fittings. Moreover, the two-material blend combination results in a considerable cost saving over three-material blends and specialty multistage polymerized HDPE pipe resins. Each of the resins in the blends can independently be virgin, recycled, reprocessed or scrap resins.
Thus, in one embodiment, the invention provides a polyethylene composition comprising a melt blend of (i) a bimodal high molecular weight high density polyethylene resin having a NCTL stress crack resistance of about 200 hours or greater, and (ii) a high density polyethylene resin selected from the group consisting of a homopolymer high density polyethylene resin, a copolymer high density polyethylene resin, and mixtures thereof, wherein the composition has a minimum NCTL stress crack resistance of 24 hours.
In another embodiment, the invention provides an extruded, molded or formed plastic article comprising the composition, and in a further embodiment provides a pipe and/or pipe fitting comprising the composition wherein the bimodal high molecular weight high density polyethylene resin and the second high density polyethylene resin are present in the polyethylene composition in amounts relative to one another such that the composition has a density of about 0.940 to about 0.960 g/cm3, preferably 0.945 to 0.955 g/cm3, and a melt flow index of about 0.05 to about 1.0, preferably about 0.1 to 0.4. The pipe and/or pipe fitting preferably has a minimum flexural modulus of 110,000 psi and a minimum tensile strength of 3,000 psi.
In another embodiment, the invention provides a polyethylene composition comprising a melt blend of (i) a bimodal high molecular weight high density polyethylene resin having a NCTL stress crack resistance of about 200 hours or greater, and (ii) a linear low density polyethylene resin (LLDPE) wherein the composition has a minimum NCTL stress crack resistance of 24 hours. Preferably, the bimodal high molecular weight high density polyethylene resin and the linear low density polyethylene resin are present in the melt blended composition in amounts relative to one another such that the composition has a density of about 0.940 to about 0.960 g/cm3, a melt flow index of about 0.1 to about 0.4. The LLDPE resin can be virgin, recycled, reprocessed and scrap resin, or mixtures of these.
The invention also provides an extruded, molded or formed plastic article comprising the HMW-HDPE and LLDPE composition.
The invention also provides methods for making the high density polyethylene compositions of the foregoing embodiments.